A spark plug is installed in a combustion chamber of an internal combustion engine of a vehicle or the like, as an ignition means for igniting a fuel-air mixture. The spark plug includes a cylindrical housing and a cylindrical insulator held on the inside of the housing. The spark plug also includes a center electrode held on the inside of the insulator with a tip end thereof being projected from the insulator, and a ground electrode forming a spark discharge gap between itself and the center electrode.
With the spark plug having the aforementioned configuration, there is a risk that radio noise is generated due to the spark discharge occurring in the spark discharge gap, and this may adversely affect the peripheral devices. To prevent such radio noise, a resistor is disposed on the base end side of the center electrode.
PTL 1 discloses the following resistor as a resistor of a spark plug to prevent radio noise. PTL 1 discloses a resistor in which the resistance in a region closer to the tip side (tip side region) than the center in the axial direction of the spark plug is made higher than the resistance in a region closer to the base end side (base end side region) than the center in the axial direction. Resistivity of the resistor in the axial direction is adjusted by appropriately adjusting, in the axial direction, the amount of carbon contained in the resistor. Namely, the resistor in the spark plug described in PTL 1 has a lower carbon content in the tip side region than in the base end side region.